Electrophotographic recording element with increased speed



3,469,979 ELECTROPHOTOGRAPHIC RECORDING ELEMENT WITH INCREASED SPEEDLouis James Ferrarini, Wakefield, R.I., Harold J. Homonolf, Saxonville,and George R. Nelson, Holliston, Mass, assignors to DennisonManufacturing Company,

Framiugham, Mass., a corporation of Nevada No Drawing. Filed Nov. 26,1965, Ser. No. 510,089

Int. Cl. G03g 7/00 US. Cl. 96--1.7 8 Claims ABSTRACT OF THE DISCLOSUREwhere each of R R R and R is either hydrogen, lower aliphatic, lowercycloaliphatic, lower acyl, aryl or heterocyclic monovalent radicals andR and R are lower divalent aliphatic radicals.

This invention relates to electrophotography and has for its principalobject provision of photoconductive recording elements and recordingprocesses with increased speed of discharge upon exposure to light.

Electrophotography is well known and involves generally theelectrostatic charging in the dark of a photoconductive layer of arecording element, and exposing the layer to a pattern of light to whichthe photoconductor is sensitive, thereby forming a latent image as anelectrostatic charge pattern in areas not receiving light. The resultingimage can be electronically scanned or displayed but is usuallydeveloped to a visible image with electroscopic colored particles. Theparticles can then be fixed, if desired, either to the recording elementitself or to a receiving surface to which they are transferred.

Recording elements comprising finely divided photoconductive particlesdispersed in insulating resinous binders are known and are disclosed inUS. Patent No. 3,121,006. Photoconductive zinc oxide in a binder isgenerally used and such recording elements are more specificallydisclosed in US. Patent No. 3,052,539. Such recording elements are knownto be sensitive to ultraviolet radiation at wavelengths of 3-800angstroms or lower. Sensitivity refers to the capacity of thephotoconductor of the recording element to dissipate electrostaticcharges on its surface by a lowering of its resistivity at specifiedwave lengths of light.

In commercial practice, it is generally preferred that the recordingelements be sensitive to light at visible Wavelengths includingSOOO to5500 angstroms. This can be accomplished by adding dye of known type asdisclosed for example in the above two patents. However, dyes or aUnited States Patent 0 3,469,979 Patented Sept. 30, 1969:

combinations of dyes which provide good sensitivity and speed to thephotoconductive layer tend to be highly colored and to impart color tothe photoconductive layer. This is a disadvantage in copying processesWhere dark prints on a white background are desired.

It has now been found that inclusion, together with sensitizing dye,within the photoconductive layer or coating of dispersed thiourea orthiourea derivatives having a carbon atom bonded to two nitrogen atomsand. one sulfur atom, materials which are not themselves sensitizingdyes, increases the speed of discharge of the coating, referred toherein as hypersensitization. In addition to the increase in speed whichit makes possible, hypersensitization permits use of a lesser amount ofdye, a greater variety of dyes, and permits formulation of zinc oxidecoatings which are more nearly White in appearance. As will appearhereinafter, the amount of the hypersensitizer additive is not criticaland the nature of the N substituents, provided they are substantiallynon-reactive in the coating, is not important, a large number includingsubstantially all types currently available having been found to beuseful.

Thus in one aspect, the present invention comprises an improvedelectrophotographic recording element comprising a backing and a coatingof finely-divided zinc oxide and finely divided sensitizing dye therefordispersed in an insulating binder wherein the improvement consists inthe dispersion within the coating of an eflective amount of ahypersensitizer for increasing the speed of discharge of the coatingwhen electrostatically charged and exposed to light, said sensitizerconsisting essentially of (a) thiourea dioxide, or (b) a compound of thestructure wherein each of R R R3 or R is selected from the groupconsisting of hydrogen, lower aliphatic, lower cycloaliphatic, loweracyl, aryl, and heterocyclic monovalent radicals and R and R are lowerdivalent aliphatic radicals. Thiourea dioxide, also known asformamidinesulforic acid, has the structure In a further aspect, theinvention comprises the method of producing a latent electrostatic imagecomprising applying a uniform electrostatic charge to the above improvedrecording element in the dark, that is, the absence of radiation towhich the photoconductor is sensitive, and exposing the element to apattern of light to which the photoconductor is sensitive.

The effect of the hypersensitizer is shown in the following examples,the compositions stated being coated onto paper 0.0022 inch in thicknessand having a resistivity at 20% relative humidity of 1000 megohms. Thecoating compositions were applied and dried to a thickness of about0.0008 inch. After storage in the dark at 20% R.H., they were charged toapproximately 500 volts using a negative corona at 7.4 kv. grounded witha positive corona of 3.0 kv. The discharge time was the time in secondsto discharge to 5 volts upon exposure to 2.0 foot candles of light froma General Electric Photo flood lamp EPR of 375 watts. The time lapsebetween charging' and exposure in each case was about 8 seconds.Voltages were measured as apparent surface voltage with the equipmentand procedure described in E. Giamo, RCA Review, volume XXII, No. 4,pages 780-790 of December 1961. All parts are by weight except whereotherwise indicated.

EXAMPLES 1-24 Zinc oxide 100. 100.0 Aeryloid 13-82 (40%) 37. 8 37.8Poly-alpha methyl styrene. 1. 4 1. 4 Toluene 70.0 70.0 sensitizing dyesolution, ml 1. 2 mercapto imidazoline. 1. 0 Hypersensitizer, one of 1.0

Exposure Example Hypersensitizer time N,N -ethylenethiourea. N methylthiourea N,N dimethyl thiourea 1,3 diethyl, 2 thiourea 1,3 dibutylthiourea N acetyl thiourea. 1 allyl, 2 thiourea 3 allyl1,1 diethyl, 2thiourea 1 phenyl, 2 thiourea 1,1 diphenyl, 2 thiourea N,N dicyclohexylthiourea. 1 cyclohexyl, 3 (2 morpholinoethyl) th10urea Thiourea 1,3 bis(carboxypentyl) thiourea N,N, bis (4 fluorophenyl) thiourea N,N, ditert-butyl thiourea N,N', di-isopropyl thiourea N,N dimethyl N phenylthiourea. N,N dimethyl N (O tolyl) thiourea N,N dimethyl N (M tolyl)thiourea. N,N dimethyl N (p tolyl) thiourea Thiourea dioxide 23-- Omethoxyphenyl thiourea 24 N,N dimethyl thiourea In the above examplesthe zinc oxide was Florence Green Seal No. 8 of the New Jersey Zinc Co.,Acryloid B-82 as a 40% solution in toluene of an acrylic polymer fromRohm and Haas Co., the alpha-methyl styrene was 4 Resin 276 V2 of theDow Chemical Company, and the sensitizing dye solution had the followingcomposition:

Eosin 0], Acid Red 87 (a hydroxyphthalein dye), 1% Calco fiuorescein,Acid Yellow 73, 1% Brom phenol blue (3,3,3,5-tetrabrorno-phenolsulphionphthalein), 1% 0.26 Brilliant Yellow 6G, 1% 0.84

The samples were prepared by dispersing the zinc oxide in the solvent,adding the resins, dye solution, and hypersensitizer, the resultingsuspension coated onto the paper base and the coating dried, for examplefor one minute at C.

To show the wide variation in useful amounts of hypersensitizeradditive, Examples 25-33 below were prepared using the Zinc oxide andresin formulation and procedure of Examples 1-24 but substituting theindicated dyes and additive in the quantity of toluene solvent indicatedin ml. Exposure time was the time in seconds to discharge to 5 voltsapparent surface voltage.

Hypersensi- Initial Exposure Solvent Dye tizer charge time The dyeemployed in Examples 25-33 was a 1% solution of Calco Florescein of theAmerican Cyanamid Co. in methyl Cellosolve, the amounts indicated beingin ml. The hypersensitizer was 2 mercapto imidazoline, the amount beingin grams. At 3 ml. of dye solution, the zinc oxide coating exhibitedsome color. While these data show that amount is not critical, Example29 with 1.0 part by weight hypersensitizer per parts zinc oxide withminimum dye is preferred.

To show that the eifect of the hypersensitizer additive is applicable toa wide variety of dye formulations, Examples 34 to 61 below wereprepared using Formula B of Examples 1 to 24 together with the dyesindicated in the following Examples 34 to 61, the dye amounts being inml. of a 1% solution of the dye in methyl Cellosolve.

EXAMPLES 34-01 Dye Class Amount Example:

3 Bromophenol blue Sulphonphthaleimuu 1,0 Eosin OJ Hydroxyphthalein. 1.0Calco fluorescein 1 0 Brilliant yellow 66 base. 1,0 Methylene blue 2. 5Auramiue O 2. 0 Azo eosine. 2. 0 Anthroquinone blue 2. 0 HLR 2.0 Acndmeorange Aerrdme. 2. 0 2. 0 2. 0 2. 0 47.- Rapidogen red CF j 48 Niagarabrilliant blue BFL. 6 0 49 Sulfidone brilliant blue 0 G cone 6, 0 50Stilbene yellow GX Stilbene 6, 0 51 Thiofiavine. Thiozole 6. 0 52Fluorescent purp Polyazo 6, 0 53 Solophenyl green.. Trisazo 6. 0 54.-Fast acid violet 10B Triarylmethane 6.0 55 Brilliant sulpho fiavmAminoketone 6. 0 56.. Ciba blue 2B Indigoid 6.0 57 Quinoline yellow baQuinoline 6.0 58 Nigrosine base Azine... 6.0 59.- Genacryl yellow SGFMethine 6. 0 60.- Zapon fast green GG Oxazine and nitro- 6. 0 61Variamine blue salt BD Azoie 6.0

In Table I below, the initial voltages obtained as stated above andfinal voltages obtained after the stated exposure time in seconds foreach of Examples 34 to 61 are given. Control figures are initial andfinal voltages for the same formulation but without the hypersensitizer.

TABLE I Control Example Exposure time Initial Final Initial Final Assensitizer, 2 mercapto imidazoline is preferred, about 1 part per 100parts zinc oxide, although amount is not critical.

It should be understood that the foregoing disclosure is for the purposeof illustration and that the invention includes all modifications withinthe scope of the appended claims.

We claim:

1. An improved electrophotographic recording element comprising abacking and a coating thereon of finely divided photoconductive zincoxide and sensitizing dyes therefor dispersed in an insulating binderwherein the improvement consists in the dispersion within the coating ofan effective amount of a hypersensitizer for increasing the speed ofdischarge of the coating when electrostatically charged and exposed tolight, said hypersensitizer selected from the group consistingessentially of (a) thiourea dioxide or (b) a compound having thestructure wherein each of R R R and R is selected from the groupconsisting of hydrogen, lower aliphatic, lower cycloaliphatic, loweracyl, aryl, and heterocyclic monovalent radicals and R and R are lowerdivalent aliphatic radicals, said heterocyclic radical consistingessentially of a six-membered ring of carbon, nitrogen and oxygen.

2. A recording element according to claim 1 wherein said backing ispaper and said hypersensitizer is selected from the group consisting ofN,N-ethylenethiourea; N methyl thiourea; N,N dimethyl thiourea; 1,3diethyl, 2 thiourea; 1,3 dibutyl thiourea; N acetyl thiourea; 1 allyl, 2thiourea; 3 allyl 1, 1 diethyl, 2 thiourea; 1 phenyl, 2 thiourea; 1,1diphenyl, 2 thiourea; N,N dicyclohexyl thiourea; 1 cyclohexyl, 3(2morpholinoethyl) thiourea; thiourea; 1,3 bis (carboxypentyl) thiourea;N,N bis (4 fluorophenyl) thiourea; N,N di tert-butyl thiourea; N,Ndi-isopropyl thiourea; N,N dimethyl N' phenyl thiourea; N,N dimethyl N(o tolyl) thiourea; N,N dimethyl N (m tolyl) thiourea; N,N dimethyl N (ptolyl) thiourea; thiourea dioxide; 0 methoxyphenyl thiourea; and N,Ndimethyl thiourea.

3. A recording element according to claim 2 wherein said hypersensitizeris present in an amount between about 0.01 and 25 parts by weight perparts zinc oxide.

4. A recording element according to claim 3 wherein said hypersensitizeris N,N-ethylene thiourea.

5. The method of producing a latent electrostatic image which comprisesapplying a uniform electrostatic charge in the dark to the coating ofthe recording element of claim 1 and exposing the coating to a patternof light.

6. The method of producing a latent electrostatic image which comprisesapplying a uniform electrostatic charge in the dark to the coating ofthe recording element of claim 2 and exposing the coating to a patternof light.

7. The method of producing a latent electrostatic image which comprisesapplying a uniform electrostatic charge in the dark to the coating ofthe recording element of claim 3 and exposing the coating to a patternof light.

8. The method of producing a latent electrostatic image which comprisesapplying a uniform electrostatic charge in the dark to the coating ofthe recording element of claim 4 and exposing the coating to a patternof light.

References Cited UNITED STATES PATENTS 3,031,301 4/1962 Agens 96-10l X3,352,670 11/1967 Coles 961.7

OTHER REFERENCES Inoue et al., Hypersensitization of Photoconduction inMicrocrystalline Zinc Oxide, Journal of Physical Chemistry, vol. 69, No.3, pp. 767-779 (March 19-65). GEORGE F. LESMES, Primary Examiner C. E.VAN HORN, Assistant Examiner U.S. Cl. X.R. 96-1

